Pyrroloquinoline quinone (PQQ), a compound first discovered in microorganisms, is a prosthetic group of a variety of important enzymes in bacteria. It also exists in the bodies of advanced eukaryotes, and distributes widely in human tissues and organs. As a tasteless and smell-less compound, PQQ has a molecular formula shown as structural formula I:

The research on PQQ in recent years shows that it is a salubrious substance for human. Mice lacking PQQ grow slowly, suffer from poor fertility, and are susceptible to arthritis, indicating that PQQ may be one of the essential vitamins in bodies. As an anti-oxidant, PQQ can protect liver from damage caused by CCl4 or alcohol. Purified PQQ can reduce the formation of glucocorticoid induced cataracta during the development of a chick embryo. PQQ protects neurocytes from the influence of NMDA toxicity, such that cerebral hypoxia-ischemia is prevented and thus severe shock of an animal model is avoided. In addition, PQQ also have functions to prevent and cure myocardial ischemia and myocardial infarction, and is effective in prevention and treatment of artherosclerosis artherosclerosis.
At present, PQQ can be prepared by way of fermentation and chemical synthesis. However, the solid forms of PQQ disodium salt prepared by these methods are unstable and highly hygroscopic. Its wetness varies greatly in the temperature range for ambient storage, and its solid state is undesirable, and thus causing negative influence on the storage and application of PQQ disodium salt.
Mitsubishi Gas Chemical Company, INC. of Japan has filed a patent application for crystal 1 (i.e. crystal E referred to in the present patent application) of PQQ disodium salt (patent literature 1, application number: CN201080031945; application date: Jun. 9, 2010). However, the present inventor has found upon research that this crystal is rather hygroscopic and thus unfavorable for processing and storage.
As such, taking into account the application of PQQ in various fields such as pharmaceuticals, health care, food, etc., there exists an urgent need for PQQ crystals which have high stability, excellent processability and storability in high purity state, and a method for preparing the same.